A negi is a type of long onion, which is somewhat in between scallion and leek. It’s used for stewing, grilling and garnish. It is a common long onion in Japan, but some people refer to it as Kanto negi, because it’s widely eaten in Tokyo area.
For some reason, it is one of the Japanese vegetables which is very difficult to find in the United States. It is sometimes sold at a Japanese grocery stores or Korean grocery stores, but tend to be over priced. So that I tend to use leeks instead for sukiyaki or other hotpot, and use scallions for garnish.
People usually cut them in half to fit in a refrigerator, because a Japanese negi is much longer than a leek. The green parts are edible, but need to be cooked.
Other than the negi in the photo above, there are several different types of long onions in Japan. Depending on the use and region, people use different types of long onions. I was surprised to see the seeds of Shimonita negi, which look more like leeks, are sold in Amazon. Shimonita negis are the best for sukiyaki.